1. Field of the Invention
The present invention relates to polarizing optical elements for use in the visible portion of the electromagnetic spectrum. More particularly, the present invention relates to broad bandwidth wire grid polarizers that efficiently transmit light of a specific polarization while efficiently reflecting light of the orthogonal polarization.
2. Prior Art
The use of an array of parallel conducting wires to polarize radio waves dates back more than 110 years. Wire grids, generally in the form of an array of thin parallel conductors supported by a transparent substrate, have also been used as polarizers for the infrared portion of the electromagnetic spectrum.
The key factor that determines the performance of a wire grid polarizer is the relationship between the center-to-center spacing, or period, of the parallel grid elements and the wavelength of the incident radiation. If the grid spacing or period is long compared to the wavelength, the grid functions as a diffraction grating, rather than as a polarizer, and diffracts both polarizations (not necessarily with equal efficiency) according to well-known principles. When the grid spacing or period is much shorter than the wavelength, the grid functions as a polarizer that reflects electromagnetic radiation polarized parallel to the grid elements, and transmits radiation of the orthogonal polarization.
The transition region, where the grid period is in the range of roughly one-half of the wavelength to twice the wavelength, is characterized by abrupt changes in the transmission and reflection characteristics of the grid. In particular, an abrupt increase in reflectivity, and corresponding decrease in transmission, for light polarized orthogonal to the grid elements will occur at one or more specific wavelengths at any given angle of incidence. These effects were first reported by Wood in 1902 (Philosophical Magazine, September 1902), and are often referred to as "Wood's Anomalies". Subsequently, Rayleigh analyzed Wood's data and had the insight that the anomalies occur at combinations of wavelength and angle where a higher diffraction order emerges (Philosophical Magazine, vol. 14(79), pp. 60-65, July 1907). Rayleigh developed the following equation to predict the location of the anomalies (which are also commonly referred to in the literature as "Rayleigh Resonances"): EQU .lambda.=.epsilon.(n.+-.sin.theta.)/k (1)
where
.epsilon. is the grating period; PA1 n is the refractive index of the medium surrounding the grating; PA1 k is an integer corresponding to the order of the diffracted term that is emerging; PA1 and .lambda. and .theta. are the wavelength and incidence angle (both measured in air) where the resonance occurs.
For gratings formed on one side of a dielectric substrate, n in the above equation may be equal to either 1, or to the refractive index of the substrate material. Note that the longest wavelength at which a resonance occurs is given by the following formula: EQU .lambda.=.epsilon.(n+sin.theta.) (2)
where n is set to be the refractive index of the substrate.
The effect of the angular dependence is to shift the transmission region to larger wavelengths as the angle increases. This is important when the polarizer is intended for use as a polarizing beam splitter or polarizing turning mirror.
FIG. 1 illustrates a basic prior art wire grid polarizer and defines terms that will be used in a series of illustrative examples of the prior art and the present invention. The wire grid polarizer 100 is comprised of a multiplicity of parallel conductive electrodes 110 supported by a dielectric substrate 120. This device is characterized by the pitch or period of the conductors, designated p; the width of the individual conductors, designated w; and the thickness of the conductors, designated t. A beam of light 130 produced by a light source 132 is incident on the polarizer at an angle .theta. from normal, with the plane of incidence orthogonal to the conductive elements. The wire grid polarizer 100 divides this beam into a specularly reflected component 140, and a non-diffracted, transmitted component 150. For wavelengths shorter than the longest resonance wavelength given by equation 2, there will also be at least one higher-order diffracted component 160. Using the normal definitions for S and P polarization, the light with S polarization has the polarization vector orthogonal to the plane of incidence, and thus parallel to the conductive elements. Conversely, light with P polarization has the polarization vector parallel to the plane of incidence and thus orthogonal to the conductive elements.
In general, a wire grid polarizer will reflect light with its electric field vector parallel to the wires of the grid, and transmit light with its electric field vector perpendicular to the wires of the grid, but the plane of incidence may or may not be perpendicular to the wires of the grid as discussed here. The geometry chosen here is for clarity of illustration.
Ideally, the wire grid polarizer will function as a perfect mirror for one polarization of light, such as the S polarized light, and will be perfectly transparent for the other polarization, such as the P polarized light. In practice, however, even the most reflective metals used as mirrors absorb some fraction of the incident light and reflect only 90% to 95%, and plain glass does not transmit 100% of the incident light due to surface reflections.
FIG. 2 shows the calculated non-diffracted, or zero-order, transmission and reflection of a prior art wire grid polarizer with the incidence angle .theta. equal to 45 degrees. These data were calculated using the Gsolver grating analysis software tool commercially available from Grating Solver Development Company, P.O. Box 353, Allen, Tex. This software tool implements the rigorous coupled wave analysis and modal methods. The analysis methods and results are similar to those reported in the literature ("Coupled-wave analysis of lamellar metal transmission gratings for the visible and the infrared", Journal of the Optical Society of America A, Vol. 12 No. 5, May 1995, pp. 1118-1127). The analysis assumes an aluminum grid with period p=0.2 .mu.m, conductor width w=0.1 .mu.m, conductor thickness t=0.1 .mu.m, and substrate refractive index n=1.525. Note that two resonances occur at wavelengths of about 0.34 .mu.m and about 0.445 .mu.m, as predicted by equation 1. Also note that these resonances only affect significantly the polarizer characteristics for P polarization.
For incident light polarized in the S direction, the performance of the prior art polarizer approaches the ideal. The reflection efficiency for S polarization is greater than 90% over the visible spectrum from 0.4 .mu.m to 0.7 .mu.m. Over this wavelength band, less than 2.5% of the S polarized light is transmitted, with the balance being absorbed. Except for the small transmitted component, the characteristics of the wire grid polarizer for S polarization are very similar to those of a continuous aluminum mirror.
For P polarization, the transmission and reflection efficiencies of the wire grid are dominated by the resonance effect at wavelengths below about 0.5 .mu.m. At wavelengths longer than 0.5 .mu.m, the wire grid structure acts as a lossy dielectric layer for P polarized light. The losses in this layer and the reflections from the surfaces combine to limit the transmission for P polarized light to about 80% over the wavelength band from 0.5 .mu.m to 0.7 .mu.m.
FIG. 3 shows the calculated performance of a different type of prior-art wire gird polarizer, as described by Tamada in U.S. Pat. No. 5,748,368. In this case, either an index matching fluid or adhesive has been used to laminate the grid structure between two substrates such that the grid is surrounded by a medium of constant refractive index. In this example, n=1.525 and the other grid parameters are unchanged from the previous example. This wire grid structure exhibits a single resonance at a wavelength about 0.52 .mu.m, as predicted by equation 1. Note that there is a narrow wavelength region, from about 0.58 to 0.62 .mu.m, where the reflectivity for P polarization is very nearly zero. U.S. Pat. No. 5,748,368 describes a wire grid polarizer that takes advantage of this effect to implement a narrow bandwidth wire gird polarizer with high extinction ratio. The examples given in the Tamada patent specification used a grid period of 550 nm, and produced a resonance wavelength from 800 to 950 nm depending on the grid thickness, conductor width and shape, and the angle of incidence. Note that the Tamada patent employs an unusual definition for polarization direction (P polarization is defined as parallel to the grid elements and thus orthogonal to the plane of incidence in defiance of the conventional definition). The resonance effect that Tamada exploits is different from the resonance whose position is predicted by Equation 1. While the two resonances may be coincident, they do not have to be. Tamada exploits this second resonance. Furthermore, there are thin film interference effects which may come into play. The bandwidth of the polarizer, where the reflectivity for the orthogonal-polarized light is less than a few percent, is typically 5% of the center wavelength. While this type of narrow band polarizer may have applications in optical memories and communications systems, many visible-light systems, such as liquid crystal displays, require polarizing optical elements with uniform characteristics over the visible-spectrum wavelengths from 400 nm to 700 nm.
Referring back to the data shown in FIG. 2, it can be seen that a necessary requirement for a wide band polarizer is that the longest wavelength resonance point must either be suppressed or shifted to a wavelength shorter than the intended spectrum of use. Referring back to equation 2, it can be seen that the wavelength of the longest-wavelength resonance point can be reduced in three ways. First, the grid period .epsilon. can be reduced. However, reducing the grid period increases the difficulty of fabricating the grid structure, particularly since the thickness of the grid elements must be maintained to ensure adequate reflectivity of the reflected polarization. Second, the incidence angle can be constrained to near-normal incidence. However, constraining the incidence angle would greatly reduce the utility of the polarizer device, and preclude its use in applications such as projection liquid crystal displays where a wide angular bandwidth centered on 45 degrees is desired. Third, the refractive index of the substrate could be lowered. However, the only cost-effective substrates available for volume manufacture of a polarizer device are several varieties of thin sheet glass, such as Corning type 1737F or Schott type AF45, all of which have a refractive index which varies between 1.5 and 1.53 over the visible spectrum.
Thus, there exists a need for an improved wire grid polarizer, particularly for use in visible light systems requiring broad wavelength bandwidth. In addition, there exists a need for such an improved wire grid polarizer for use at incidence angles of about 45 degrees. Specifically, there is a need for a polarizer structure in which the longest-wavelength resonance point can be eliminated or shifted to a shorter wavelength.